Left ventricular (LV) volumes are important prognostic indices in patients with heart disease. Although several methods can evaluate LV volumes, most have important intrinsic limitations. Real-time three-dimensional echocardiography (RT3D echo) is a novel technique capable of instantaneous acquisition of volumetric images. This study was undertaken to validate rapid LV volume calculations with RT3D echo and to determine their usefulness in cardiac patients. Hearts explanted from 11 sheep were studied ex vivo. A latex balloon was positioned within the left ventricle and filled with known amounts of saline solution (n=52; range: 38-120 ml). Volumetric images acquired with RT3D echo were compared to the actual LV volumes. In addition, 4 normals and 21 cardiac patients underwent magnetic resonance imaging (MRI) and RT3D echo on the same day. In the ex vivo animal studies, a strong correlation was observed between the actual LV volumes and those calculated with RT3D echo (r=0.99; y=1.31+0.98 x; SEE=2.2 ml). In the human studies, a strong correlation was found between LV volumes calculated with MRI and with RT3D echo (r=0.91; y=20.1+0.71x; SEE=28 ml). LV volumes obtained with MRI were greater than those obtained with RT3D echo (126+/-83 vs. 110+/-65 ml; p=0.002), probably due to the fact that the heart rate during MRI acquisition was lower than that during RT3D echo examination (62+/-11 vs. 79+/-16 beats/min; p=0.0001). Analysis of intra and interobserver variability showed strong indices of agreement in the measurement of LV volumes with RT3D echo. Thus, LV volume measurements with RT3D echo are accurate and reproducible. This technique expands the use of ultrasound for the non-invasive evaluation of patients with heart conditions and provides a new tool for the investigational study of cardiovascular disease.